1. Field of the Invention
The present invention relates to a device for the processing and pre-correction of an audio signal before it is amplified in an amplification system of a transmitter with amplitude modulation.
It can be applied notably to the making of transmitters with amplitude modulation, comprising pulse-width modulation systems.
2. Description of the Prior Art
In these transmitters, the audio signal is converted into digital samples to be applied as described in the French patent No. 84 10806 (2 567 338), to a logic unit for the control of modules that generate pulses of modulated width, said modules being series-connected to the input of a filter. The control logic unit associates the modules in series as a function of the amplitude of digital samples applied to the control logic unit in order to reconstitute the modulation signal at the output of the filter. However, since the pulse-generating modules are formed by current-chopping semiconductor devices, a distortion of the audio signal appears at the output of the filter. This distortion is due chiefly to the switch-over frequency of the generating modules, and this distortion produces parasitic lines in the audio spectrum transmitted. To attenuate this distortion, the switch-over frequency of the generating modules is chosen so as to be relatively high with respect to the frequency of the audio signal and the sampling and digitization frequencies of the audio signal are determined so as to be higher than the switch-over frequency of the modules. However, these arrangements prove to be insufficient, and there is always a fairly substantial distortion, especially for the high rates of modulation.
The aim of the invention is to overcome the above-mentioned drawbacks.